1. Field of the Invention
The present invention relates generally to a device which is able to monitor its own number of actuations and predict its own end of life and, more particularly, it relates to a limit switch which is able to count its own actuation and store the value in a way which reduces the number of lost actuations while working within the capabilities of its constituent components.
2. Description of the Prior Art
In many applications, devices are used to control the operation of machinery, assembly lines or other types of equipment. A typical example of this type of device is the limit switch which is very well known to those skilled in the art. In operation, the limit switch is typically located proximate the path of an object being manipulated by the equipment or, alternatively, proximate the path of a portion of the equipment. When the limit switch is actuated, it typically makes or breaks electrical power between two preselected locations. In addition, limit switches can be used to provide a signal to a controller that a preselected event has occurred. These and many other applications of limit switches are well known to those skilled in the art.
Like most mechanical devices, limit switches have a certain lifetime after which it can be expected that the switch will fail for some mechanical reason. For example, components within the limit switch can wear out due to normal frictional effects. In addition, components may fail because of repeated flexing such as when a spring is cycled. Another possible cause for a switch failure is contact erosion that occurs between a moveable contact and a stationary contact. These and other causes can lead to the failure of a switch after a certain number of operations. Empirical studies can determine the expected life of a device based on statistical information obtained from many thousands of devices. Since the load across the contacts of limit switches in many applications is either controlled or limited, it is possible to predict the expected life of a switch by using load-life curves that describe the expected number of actuations in the life of a switch as a function of the current load on the switch at a given voltage potential. By using curves like these, the expected life of a limit switch can be predicted and a reasonable time for replacement can be determined so that catastrophic failure of the switch is avoided. This information can be used to predict the expected life of the switch.
If a means is provided for counting the actuations of a device, such as a limit switch, the number of actuations performed during the life of the switch can then be used as a criterion for determining when the device should be replaced as part of a preventative maintenance program. However, previous attempts to monitor the number of actuations of devices such as limit switches have proven to be difficult and counterproductive. For example, the number of operations of a limit switch can be counted by another mechanical device, but that device will also experience the normal reasons for failure, such as frictional wear. Therefore, the device used to count the operations of the limit switch may fail before the limit switch itself. Electronic means can be used to count the actuations of a limit switch, but this technique also exhibits certain inherent problems. For example, if volatile storage devices are used, a power failure can cause the stored information to be lost. If a nonvolatile storage device is used, the number of data storage operations is often limited to a certain maximum limit and the permitted number of storage operations may, in fact, be less than the expected lifetime of the switch that is being monitored.
Another problem related to the monitoring of switch actuations is the costs. In some situations, the costs of monitoring the number of operations of a limit switch can exceed the costs of the switch itself and therefore make the effort counterproductive. In addition, when devices such as limit switches are monitored to maintain a count of their actuations, it is necessary to determine the number of actuations in order to decide on when preventive maintenance replacement should occur. It is sometimes difficult to determine the precise count stored in the limit switch and inform the user of the number of actuations. The replacement of the switch is sometimes less expensive than the determination of the number of counts stored at any given time.
In view of the above, few attempts have been made to monitor the life of a limit switch even though the benefits are widely recognized. If a means is provided to facilitate the monitoring of the number of actuations of a mechanical device such as a limit switch, significant reduction in operating costs can be achieved. For example, if a complex assembly line or piece of equipment utilizes hundreds of limit switches, it might be extremely expensive to shut down the assembly line or piece of equipment for the purpose of replacing a limit switch that has failed during operation. This type of catastrophic failure can result in costly downtime or possible damage to equipment. It would be very beneficial if, during a required shutdown to replace a limit switch, all of the other switches on the assembly line or piece of equipment could be quickly and easily examined to determine whether they are also approaching their end of life. For those switches that have not reached their end of life number of actuations, but are close to that number, they can be replaced during the same shutdown of the assembly line or piece of equipment as part of a preventative maintenance program.